1. Field of the Invention
The present invention relates generally to fluid control valves and more particularly to hydraulic or pneumatic control valves of the flapper type for controlling the resultant pressure within a pressure chamber from two sources of pressure in proportion to an electrical control signal.
The control valve of the present invention while having general application in controlling fluid pressures, is particularly applicable in pneumatic pressure control systems such as pneumatic test apparatus for testing the various pneumatic pressure systems of aircraft. For example, it is useful in ground testing aircraft air data systems which in actual use provide aircraft control and display avionics in accordance with measures of the aircraft's altitude, vertical speed, airspeed, mach number, etc. Such test apparatus must therefore be capable of precisely duplicating pneumatic pressures on the ground normally encountered by an aircraft in flight over its entire flight profile. Typical of such pneumatic test apparatus is that disclosed in the present inventor's U.S. patent application Ser. No. 735,249, filed Oct. 26, 1976 now U.S. Pat. No. 4,086,804 entitled "Improved Pneumatic Pressure Supply System" and assigned to the same assignee as the present application. As disclosed therein, the desired pneumatic pressure and/or pressure rate to be supplied to the aircraft pneumatic pressure equipment under test is derived from a controlled or load pneumatic pressure volume through suitable pneumatic lines connected, for example, to the aircraft pneumatic sensors (which, of course, becomes part of the load volume). The pressure in the load volume is precisely controlled through a digital outer control loop servo and an inner analog electro-pneumatic closed loop servo. In the latter loop, the pressure in the volume is detected and converted into an electrical signal which signal is electrically summed with a pressure command signal from the digital outer loop, the resultant signal energizing an electrically actuated pressure control valve which in turn controls the pressure in the load volume (and in the aircraft pneumatic equipment) to maintain the digital electrical error signal zero, that is, the load volume pressure is maintained equal to that commanded. As disclosed in the above application, the control valve is of the flapper type wherein a flapper is electrically positioned in a gap defined by two nozzles, one connected to a source of positive pressure and the other to a source of negative pressure, e.g., a vacuum pump. The electric signal positions the flapper valve in the gap so as to control the amount of gas supplied to or withdrawn from the load volume to maintain the desired pressure therein.
As described in the above application, the pneumatic test equipment is a two channel system, one for supplying a controlled aircraft static pressure P.sub.s and one for supplying a controlled aircraft total pressure P.sub.t (dynamic plus static). In supplying steady state test pneumatic pressures corresponding, for example, to a very high altitude or a very high airspeed, it will be appreciated that very low and very high pressures respectively will have to be supplied and controlled. The flapper valves of the type schematically illustrated in the above application and in FIG. 6 of the present application, suffer from a design deficiency which, when called upon to control and maintain a steady state pressure, for example, incurs a large mass flow of air (or gas) through the valve and therefore an expensive large, high capacity vacuum pump is required. This quiescent large mass flow is wasted and if the positive pressure source is dry air or dry nitrogen, such "wasted" mass flow is very expensive. Further, with presently known flapper valve designs which inherently incur large mass flow, two large capacity vacuum pumps are required for air data test equipment, one for P.sub.s and one for P.sub.t. If the wasted mass flow could be substantially reduced, only one vacuum pump would be required for both parameters in many applications. This is particularly desirable with portable or flight line test pneumatic equipment. Also, with a valve design which reduces quiescent wasted mass flow, the test equipment user can size his vacuum pump based on the aircraft pneumatic system volume and flight profile of the aircraft under test rather than on the mass flow requirements of the test equipment itself. As the orifice sizes and strokes are increased to increase the transient mass flow capability such as required by a flight line test system, the quiescent mass flow can be reduced.